1. Field of the Invention
The present invention relates to a nitride semiconductor surface emitting laser and a method of manufacturing the same, and especially related to a nitride semiconductor surface emitting laser including a plurality of active layers and a method of manufacturing the same.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is a laser in which an active layer is sandwiched by two reflectors on a substrate and a cavity is formed in the vertical direction (EP 1081816 A2).
The reflectance of a semiconductor DBR (multilayer film reflector) that configures the cavity is raised to 99.5% or more and a cavity loss is decreased, so that laser oscillation is realized.
In the VCSEL, the thickness of the active layer is extremely thinner than that of the cavity. Therefore, structurally, it is less likely to obtain a gain, and it is difficult to realize a high output.
Therefore, as a technology to increase the gain, a periodic gain structure in which a plurality of active layers is arranged at peak positions of a stationary wave in the cavity has been proposed.
When the periodic gain structure is used in the VCSEL of the nitride semiconductor, it is necessary to make an intermediate layer to be formed between the plurality of active layers into p-type in order to inject carriers into all of the plurality of active layers.
For example, when the intermediate layer is i-type or n-type, electrons to be injected from a side of an n-type layer is first injected into an active layer adjacent to the n-type layer.
Since the electron has large mobility, the electrons are leaked out from the active layer adjacent to the n-type layer, and are injected into a neighbor active layer and are injected into a further neighbor active layer.
Meanwhile, the mobility of a hole is smaller than that of the electron by 1 to 2 digits. Therefore, when holes are injected from a p-type layer, most of the holes are subjected to light emission recombination in the active layer adjacent to the p-type layer, and the holes are not injected into a neighbor active layer.
A gain is only caused in the active layer adjacent to the p-type layer even if a plurality of active layers is used, and other active layers become absorption layers.
Therefore, all intermediate layers to be formed between the active layers are made into the p-type. The intermediate layers are made into the p-type, so that the holes exist in all active layers in high concentration.
Since the electron is lighter than the hole, and is easily injected deep into a p-type layer, the carriers can be injected into all of the active layers.